The present invention relates generally to furnace apparatus and, in a preferred embodiment thereof, more particularly provides an induced draft, fuel-fired furnace having incorporated therein a bypass system for substantially preventing the combustion products from a standing pilot flame within the furnace from internally migrating through the furnace heat exchanger, and potentially corroding it, during idle periods of the furnace.
The National Appliance Energy Conservation Act of 1987 mandates that all forced air furnaces produced after Jan. 1, 1992, and having heat output ratings of from 45,000 to 400,000 Btuh, must have a minimum heating efficiency of 78% based upon Department of Energy test procedures. The increased stringency of this new requirement imposed upon furnace manufacturers has prompted a considerable amount of redesign effort, since most forced air furnaces of this type currently being manufactured do not meet the upcoming heating efficiency standards.
One furnace modification currently being investigated in an attempt to meet these higher heating efficiency standards is the provision of an improved efficiency furnace heat exchanger through which the main burner system combustion products are flowed to transfer heat to supply air forced externally across the heat exchanger. The typical heat exchanger presently utilized in forced air furnaces of this type is of a relatively large "clamshell" configuration which has a relatively low resistance to combustion product flow therethrough.
In conjunction with this conventional clamshell type heat exchanger, a standing pilot flame is continuously maintained within the furnace housing to ignite fuel discharged from the main burner structure when the furnace experiences a demand for heat from its associated space thermostat. As is well known, the structure used to create and maintain such standing pilot flame is quite simple and reliable, is relatively inexpensive, and is easy to install and replace. In the conventional atmospheric furnace, the continuously generated combustion products from the standing pilot flame are permitted to vent through the clamshell heat exchanger into an atmospheric exhaust stack during idle periods of the furnace. Due to the relatively low flow pressure drop characteristics of the typical clamshell heat exchanger, the pilot flame combustion products relatively quickly traverse its interior without creating an excessive amount of corrosive condensation therein.
A method being currently being considered for improving furnace heat exchanger efficiency is to increase the heat exchanger internal flow pressure drop and connect the "tighter" heat exchanger to a draft inducer fan which is operative to forcibly draw the main burner combustion products through the heat exchanger and discharge them into the typically provided exhaust stack during heating cycles of the furnace. However, as the internal pressure drop of a heat exchanger is increased, the potential for internal heat exchanger corrosion caused by migration therethrough of standing pilot flame combustion products during idle periods of the furnace is correspondingly increased. This is due to the fact that such pilot flame combustion products are maintained for longer periods of time within the "tighter" heat exchanger, during idle periods of the furnace, thereby creating considerably more corrosive condensation within the heat exchanger.
The result of this heretofore unavoidable trade-off between the enchanced thermal efficiency of a higher pressure drop heat exchanger and heightened heat exchanger corrosion has typically required that the simple and relatively inexpensive standing pilot flame structure be eliminated and replaced with a considerably more complex and expensive electric ignition system. This previously necessary use of the costlier and more complex electric ignition system in conjunction with high pressure drop heat exchangers in induced draft furnaces has correspondingly increased their manufacturing and maintenance costs and added to their structural and operational complexity.
From the foregoing it can be seen that it would be quite desirable to provide an induced draft, fuel-fired furnace which is provided with a high efficiency, relatively high pressure drop heat exchanger, while at the same time utilizing a standing pilot flame system without creating the aforementioned heat exchanger corrosion problems during idle periods of the furnace. It is accordingly an object of the present invention to provide such a furnace.